Tread for a tire

ABSTRACT

A tire tread comprising a rubber composition based on at least: (i) a diene elastomer; (ii) more than 60 phr of a reinforcing inorganic filler; (iii) between 2 and 15 phr of a coupling agent; and (iv) between 4 and 12 phr of a bismaleimide compound. This tread has, after mechanical running-in of the tire comprising it, a rigidity gradient which increases radially from the surface towards the inside of the tread, imparting to the tire an improved compromise of grip/behaviour properties. Use of such a tread for the manufacturing or recapping of tires. Tires comprising such a tread.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of PCT/EP03/01126 filed on 5 Feb.2003, and published in the French language on 14 Aug. 2004, asWO2003/066722. The present application also claims priority to Frenchnational filing of 7 Feb. 2002, serial number 02/01659.

FIELD OF THE INVENTION

The present invention relates to treads for tires and to rubbercompositions used for the manufacture of such treads.

BACKGROUND OF THE INVENTION

It relates more particularly to treads for tires having a low rollingresistance, reinforced majoritarily by inorganic fillers, these treadsbeing intended in particular for tires fitted on vehicles such asmotorcycles, passenger cars, vans or heavy vehicles.

Since fuel economies and the need to protect the environment have becomepriorities, it has proved necessary to produce tires having both reducedrolling resistance and high wear resistance. This has been made possibledue in particular to the discovery of new rubber compositions reinforcedwith specific inorganic fillers referred to as “reinforcing” fillers,which are capable of rivalling conventional carbon black from thereinforcing point of view, and furthermore offering these compositions alow hysteresis, which is synonymous with lesser rolling resistance forthe tire treads comprising them. Such compositions based on reinforcinginorganic fillers of the siliceous or aluminous type, have for examplebeen described in patents or patent applications EP-A-0 501 227, EP-A-0735 088, EP-A-0 810 258, EP-A-0 881 252, WO99/02590, WO99/02601,WO99/02602, WO99/28376, WO00/05300, WO00/05301, WO01/96442, WO02/30939,WO02/31041 and WO02/083782.

However, ideally, a tire tread must meet other technical demands, someof which are contradictory, such as having in particular very good gripboth on dry ground and on wet, snow-covered or icy ground, whileoffering the tire a very good level of road behavior (“handling”) on anautomobile, in particular high drift thrust (or “cornering”).

To improve the road behavior, it is known that greater rigidity of thetread is desirable, this stiffening possibly being obtained for exampleby increasing the amount of reinforcing filler or by incorporatingcertain reinforcing resins into the rubber compositions constitutingthese treads.

However, such stiffening of the tread, at the very least for its surfacepart which is in contact with the ground during rolling of the tire isknown to impair, most frequently in crippling manner, the properties ofgrip on wet, snow-covered or icy ground.

This is why, in order to meet these two contradictory demands, namelyroad behaviour and grip, it has essentially been proposed hitherto touse composite treads (i.e. hybrid treads), formed by two radiallysuperposed layers (“cap-base structure”) of different rigidities, formedof two rubber compositions of different formulations: the radially outerlayer, in contact with the road, is formed of the more flexiblecomposition, in order to meet the grip requirements; the radially innerlayer is formed of the more rigid composition, in order to meet the roadbehavior requirements.

Such a solution however has numerous disadvantages:

-   -   first of all, the manufacturing of a composite tread is by        definition more complex and therefore more costly than that of a        conventional tread, and requires in particular the use of        complex coextrusion machines;    -   during manufacturing, after cutting out the tread to the correct        dimensions once it has emerged from the extruder, it is        furthermore necessary to manage discarding of material of        different natures, which further substantially increases the        production costs;    -   finally, and this is not the least of the disadvantages, once        the radially outer (flexible) part of the tread has become worn,        it is the initially inner part of the tread which comes into        contact with the road: then, of course, one has the        disadvantages of an excessively rigid tread, with unsatisfactory        performance from the point of view of the technical compromise        initially intended.

SUMMARY OF THE INVENTION

Now, the Applicant has discovered during his research that a specificrubber composition, based on a high amount of reinforcing inorganicfiller and a bismaleimide compound, makes it possible, owing to anunexpected “auto-accommodation” phenomenon, to obtain a tread having atrue rigidity gradient, radially increasing from the surface towards theinside of the tread. This rigidity gradient is obtained not only simplyand economically, but also durably, thus making it possible to maintainthe compromise between grip and road behavior of the tires at a veryhigh level, throughout the life of the latter.

Consequently, a first subject of the invention relates to a tire treadcomprising a rubber composition based on at least (phr=parts by weightper hundred parts of elastomer):

-   -   (i) a diene elastomer;    -   (ii) more than 60 phr of a reinforcing inorganic filler;    -   (iii) between 2 and 15 phr of a coupling agent;    -   (iv) between 4 and 12 phr of a bismaleimide compound.

Bismaleimide compounds are well-known to the person skilled in the artfor their ability to form, by polymerization, a three-dimensionalreinforcing resin lattice (hereafter “bismaleimide lattice”) upon acuring operation. They have been used in rubber compositions, inparticular for tires, for applications as varied as adhesion,reinforcement, tear resistance and vulcanization. For a detaileddescription of these compounds, the processes for obtaining them ortheir various applications, in particular in tire rubber applications,reference may be made to the following documents: FR-A-1 257 913, FR-A-2611 209, EP-A-0 345 825, EP-A-0 410 152, EP-A-0 475 222, EP-A-0 536 701,EP-A-0 564 966, EP-A-1 083 199, US-A-2,289,504, U.S. Pat. No. 3,219,091,U.S. Pat. No. 4,803,250, U.S. Pat. No. 4,818,601, U.S. Pat. No.5,109,055, U.S. Pat. No. 5,153,248, U.S. Pat. No. 5,262,488, U.S. Pat.No. 5,300,585, U.S. Pat. No. 5,985,963, Japanese applications publishedunder the Nos. JP1989/62338, JP1989/278543 and JP1991/54235.

However, as far as the Applicants is aware, no document of the prior artdescribes the use in a tire tread, in the proportions set forth here(between 4 and 12 phr), of a bismaleimide compound in combination withsuch a high amount (more than 60 phr, preferably more than 70 phr) of areinforcing inorganic filler such as silica. The knowledge of the personskilled in the art, acquired essentially in the field of rubbercompositions filled majoritarily with carbon black, on the contrarypointed him away from such a use, owing to a stiffening ability, forthis type of bismaleimide compounds, which is deemed a greatdisadvantage with regard in particular to the grip properties mentionedabove. To illustrate this prejudice, reference will be made for exampleto the aforementioned application EP-A-1 083 199, the examples ofembodiment of which are limited to amounts of bismaleimide compoundswithin a range from 1 to 4 phr, whatever the type of reinforcing fillerused.

The subject of the invention is also the use of such a tread for themanufacturing of new tires or the recapping of worn tires. The treadaccording to the invention is particularly suited to tires intended tobe fitted on passenger vehicles, 4×4 vehicles (having 4 driving wheels),motorcycles, vans and heavy vehicles (i.e. subway trains, buses, roadtransport machinery, off-road vehicles).

The subject of the invention is also these tires themselves when theycomprise a tread according to the invention. It relates in particular totires of “winter” type intended for snow-covered or icy roads.

Another subject of the invention is a process for preparing a tiretread, having, after curing and mechanical running-in of the tirecomprising it, a rigidity gradient which increases radially from thesurface towards the inside of the tread, this process beingcharacterized in that it comprises the following steps:

-   -   incorporating in a diene elastomer, in a mixer, during a first        step referred to as “non-productive”:        -   more than 60 phr of a reinforcing inorganic filler;        -   between 2 and 15 phr of a coupling agent; and        -   between 4 and 12 phr of a bismaleimide compound,    -    thermomechanically kneading the entire mixture, in one or more        stages, until a maximum temperature of between 130° C. and        200° C. is reached;    -   cooling the entire mixture to a temperature of less than 100°        C.;    -   then incorporating, during a second step referred to as        “productive”:        -   a vulcanization system;    -   kneading the entire mixture until a maximum temperature less        than 120° C. is reached; and    -   extruding or calendering the rubber composition thus obtained,        in the form of a tire tread.

DETAILED DESCRIPTION OF THE INVENTION

The invention and its advantages will be readily understood in the lightof the description and the examples which follow.

1. Measurements and Tests Used

The treads and rubber compositions constituting these treads arecharacterized or tested as indicated hereafter.

I-1. Shore A Hardness

The Shore A hardness of the compositions after curing is assessed inaccordance with ASTM Standard D 2240-86.

I-2. Tensile Tests

The tensile tests make it possible to determine the elasticity stressesand the properties at break, after curing. Unless indicated otherwise,they are effected in accordance with French Standard NF T 46-002 ofSeptember 1988. The nominal secant moduli (or apparent stresses, in MPa)at 10% elongation (ME10), 100% elongation (ME100) and 300% elongation(ME300) are measured in a second elongation (i.e. after a cycle ofaccommodation to the amount of extension provided for the measurementitself).

The nominal secant modulus is also measured at 10% elongation, after anaccommodation of 15% (i.e. an extension to 15% followed by relaxation to0%) instead of 10% as previously for the modulus ME10. This so-called“accommodated” modulus is referred to as ME10_(Ac). All these tensilemeasurements are effected under normal conditions of temperature andhumidity (23±2° C. and 50±5% relative humidity, in accordance withFrench standard NF T 40-101 of December 1979).

I-3. Mechanical Accommodation

“Mechanical accommodation” is understood to mean simple running-in ofthe tire by means of which its tread is placed in contact with theground during rolling, that is to say in working conditions, for severaltens of seconds or several minutes at the most. This running-inoperation may be carried out on an automatic rolling machine or directlyon an automobile; it may be effected in various ways, for example bysimply rolling in a straight line of several tens or hundreds of meters,by longitudinal braking or alternatively by drifting of the tire(bends), the important thing being to start making the tread “work”under normal conditions of use.

By way of example, such mechanical accommodation may be achieved by whatis called “standard” running-in consisting of simple running in astraight line over a length of 400 meters at a speed of 60 km/h, on agiven automobile, without drifting or cambering imposed on the tire,followed by moderate longitudinal braking (braking distance from 30 to40 meters) to stop the vehicle. This standard running-in is furthermoreeffected under normal conditions of pressure (those recommended by themanufacturer of the vehicle used) and load (1 person only on board thevehicle).

II. Detailed Description of Embodiments

The treads according to the invention are formed, at least in part, of arubber composition based on at least: (i) at least one diene elastomer;(ii) a minimum quantity (more than 60 phr) of at least one inorganicfiller as reinforcing filler; (iii) at least one coupling agent (between2 and 15 phr) providing the bond between the reinforcing inorganicfiller and this diene elastomer; and (iv) at least one bismaleimidecompound (between 4 and 12 phr).

Of course, the expression composition “based on” is to be understood tomean a composition comprising the mix and/or the product of reaction insitu of the various constituents used, some of these base constituents(for example the reinforcing inorganic filler, the coupling agent, thebismaleimide compound) being liable to, or intended to, react together,at least in part, during the different phases of manufacturing of thetreads, in particular during the vulcanization or curing thereof.

In the present description, unless expressly indicated otherwise, allthe percentages (%) indicated are mass %.

II-1. Diene Elastomer

“Diene” elastomer or rubber is understood to mean, generally, anelastomer resulting at least in part (i.e. a homopolymer or a copolymer)from diene monomers (monomers bearing two double carbon-carbon bonds,whether conjugated or not). “Essentially unsaturated” diene elastomer isunderstood here to mean a diene elastomer resulting at least in partfrom conjugated diene monomers, having a content of members or units ofdiene origin (conjugated dienes) which is greater than 15% (mol %).Thus, for example, diene elastomers such as butyl rubbers or copolymersof dienes and of alpha-olefins of the EPDM type do not fall within thisdefinition, and may on the contrary be described as “essentiallysaturated” diene elastomers (low or very low content of units of dieneorigin which is always less than 15%). Within the category of“essentially unsaturated” diene elastomers, “highly unsaturated” dieneelastomer is understood to mean in particular a diene elastomer having acontent of units of diene origin (conjugated dienes) which is greaterthan 50%.

These general definitions being given, the person skilled in the art oftires will understand that the present invention is used first andforemost with highly unsaturated diene elastomers, in particular with:

-   (a) any homopolymer obtained by polymerization of a conjugated diene    monomer having 4 to 12 carbon atoms;-   (b) any copolymer obtained by copolymerization of one or more    conjugated dienes with each other or with one or more vinyl-aromatic    compounds having 8 to 20 carbon atoms.

Suitable conjugated dienes are, in particular, 1,3-butadiene,2-methyl-1,3-butadiene, 2,3-di(C₁-C₅)alkyl-1,3-butadienes such as, forinstance, 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene,2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene, anaryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene. Suitablevinyl-aromatic compounds are, for example, styrene, ortho-, meta- andpara-methylstyrene, the commercial mixture “vinyltoluene”, para-tert.butylstyrene, methoxystyrenes, chlorostyrenes, vinylmesitylene,divinylbenzene and vinylnaphthalene.

The copolymers may contain between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinyl-aromatic units. The elastomersmay have any microstructure, which is a function of the polymerizationconditions used, in particular of the presence or absence of a modifyingand/or randomizing agent and the quantities of modifying and/orrandomizing agent used. The elastomers may for example be block,statistical, sequential or microsequential elastomers, and may beprepared in dispersion or in solution; they may be coupled and/orstarred or alternatively functionalized with a coupling and/or starringor functionalizing agent.

Preferred are polybutadienes, and in particular those having a contentof 1,2-units of between 4% and 80%, or those having a content of cis-1,4of more than 80%, polyisoprenes, butadiene/styrene copolymers, and inparticular those having a styrene content of between 5% and 50% byweight and, more particularly, between 20% and 40%, a content of1,2-bonds of the butadiene fraction of between 4% and 65%, and a contentof trans-1,4 bonds of between 20% and 80%, butadiene/isoprene copolymersand in particular those having an isoprene content of between 5% and 90%by weight and a glass transition temperature (“Tg”—measured inaccordance with ASTM Standard D3418-82) of −40° C. to −80° C.,isoprene/styrene copolymers and in particular those having a styrenecontent of between 5% and 50% by weight and a Tg of between −25° C. and−50° C. In the case of butadiene/styrene/isoprene copolymers, thosewhich are suitable are in particular those having a styrene content ofbetween 5% and 50% by weight and, more particularly, between 10% and40%, an isoprene content of between 15% and 60% by weight, and moreparticularly between 20% and 50%, a butadiene content of between 5% and50% by weight, and more particularly between 20% and 40%, a content of1,2-units of the butadiene fraction of between 4% and 85%, a content oftrans-1,4 units of the butadiene fraction of between 6% and 80%, acontent of 1,2- plus 3,4-units of the isoprene fraction of between 5%and 70%, and a content of trans-1,4 units of the isoprene fraction ofbetween 10% and 50%, and more generally any butadiene/styrene/isoprenecopolymer having a Tg of between −20° C. and −70° C.

In summary, particularly preferably, the diene elastomer of thecomposition used in the tread according to the invention is selectedfrom the group of highly unsaturated diene elastomers constituted bypolybutadienes (BR), synthetic polyisoprenes (IR), natural rubber (NR),butadiene copolymers, isoprene copolymers and mixtures of theseelastomers. Such copolymers are more preferably selected from the groupwhich consists of butadiene/styrene copolymers (SBR), butadiene/isoprenecopolymers (BIR), isoprene/styrene copolymers (SIR),isoprene/butadiene/styrene copolymers (SBIR) and mixtures of suchcopolymers.

The tread according to the invention is preferably intended for apassenger-car tire. In such a case, the diene elastomer is preferably anSBR copolymer, in particular an SBR prepared in solution, preferablyused in a mixture with a polybutadiene; more preferably, the SBR has acontent of styrene of between 20% and 30% by weight, a content of vinylbonds of the butadiene fraction of between 15% and 65%, a content oftrans-1,4 bonds of between 15% and 75% and a Tg of between −20° C. and−55° C., and the polybutadiene has more than 90% cis-1,4 bonds.

The compositions of the treads of the invention may contain a singlediene elastomer or a mixture of several diene elastomers, the dieneelastomer(s) possibly being used in association with any type ofsynthetic elastomer other than a diene one, or even with polymers otherthan elastomers, for example thermoplastic polymers.

II-2. Reinforcing Inorganic Filler

“Reinforcing inorganic filler” is to be understood in known manner tomean any inorganic or mineral filler, whatever its colour and its origin(natural or synthetic), also referred to as “white” filler or sometimes“clear” filler, in contrast to carbon black, which is capable, on itsown, without any other means than an intermediate coupling agent, ofreinforcing a rubber composition intended for the manufacture of a tiretread, in other words which is capable of replacing a conventionaltire-grade carbon black (for treads) in its reinforcement function.

Preferably, the reinforcing inorganic filler is a filler of thesiliceous (for example silica) or aluminous (for example alumina) type,or a mixture of these two types of fillers.

The silica (SiO₂) used may be any reinforcing silica known to the personskilled in the art, in particular any precipitated or fumed silicahaving a BET surface area and a CTAB specific surface area both of whichare less than 450 m²/g, preferably from 30 to 400 m²/g. Highlydispersible precipitated silicas (referred to as “HDS”) are preferred,in particular when the invention is used for the manufacture of tireshaving a low rolling resistance; “highly dispersible silica” isunderstood in known manner to mean any silica having a substantialability to disagglomerate and to disperse in an elastomeric matrix,which can be observed in known manner by electron or optical microscopyon thin sections. As examples of such preferred HD silicas, mention maybe made of the silicas Ultrasil 7000 and Ultrasil 7005 from Degussa, thesilicas Zeosil 1165 MP, 1135 MP and 1115 MP from Rhodia, the silicaHi-Sil EZiSOG from PPG, the silicas Zeopol 8715, 8745 and 8755 fromHuber, and treated precipitated silicas such as, for example, thealuminium-“doped” silicas described in the aforementioned applicationEP-A-0 735 088.

The reinforcing alumina (Al₂O₃) preferably used is a highly dispersiblealumina having a BET surface area from 30 to 400 m²/g, more preferablybetween 60 and 250 m²/g, an average particle size at most equal to 500nm, more preferably at most equal to 200 nm, as described in theaforementioned application EP-A-0 810 258. Examples of such reinforcingaluminas are in particular the aluminas “Baikalox”, “A125” or “CR125”(from Baïkowski), “APA-100RDX” (from Condea), “Aluminoxid C” (fromDegussa) or “AKP-G015” (Sumitomo Chemicals). The invention can also beimplemented by using as reinforcing inorganic filler the specificaluminium (oxide-)hydroxides such as described in WO99/28376.

The physical state in which the reinforcing inorganic filler may bepresent is immaterial, whether it be in the form of a powder,microbeads, granules, pellets, balls or any other appropriate densifiedform.

Of course, “reinforcing inorganic filler” is also understood to meanmixtures of different reinforcing inorganic fillers, in particular ofhighly dispersible siliceous and/or aluminous fillers such as describedabove.

When the treads of the invention are intended for tires of low rollingresistance, the reinforcing inorganic filler used, in particular if itis silica, preferably has a BET surface area of between 60 and 250 m²/g,more preferably between 80 and 230 m²/g.

The inorganic filler used as reinforcing filler must be present in ahigh amount, greater than 60 phr, preferably greater than 70 phr, whichis one of the essential characteriztics of the invention, thisreinforcing inorganic filler possibly constituting all or the majorityof the total reinforcing filler, in this latter case associated forexample with a minority quantity of carbon black (preferably less than20 phr, more preferably less than 15 phr).

The person skilled in the art will readily understand that the optimumamount will differ according to the nature of the reinforcing inorganicfiller used and the type of tire in question, for example tire formotorcycles, for passenger vehicles or alternatively for utilityvehicles such as vans or heavy vehicles. Preferably, the amount ofreinforcing inorganic filler is between 70 and 120 phr, more preferablystill within a range from 80 to 110 phr approximately, for example from80 to 105 phr in the specific case of treads for passenger-vehicletires.

Preferably, in the tread according to the invention, the reinforcinginorganic filler constitutes more than 80% by weight of the totalreinforcing filler, more preferably more than 90% by weight (or evenall) of this total reinforcing filler. However, without significantlyaffecting the technical effect desired, a small quantity of carbonblack, preferably less than 20%, more preferably still less than 10% byweight relative to the quantity of total reinforcing filler, may beused.

The carbon black, if used, is preferably present in an amount of between2 and 15 phr, more preferably between 4 and 12 phr. It can be used inparticular as a simple black pigmentation agent, or alternatively toprotect the tread from different sources of atmospheric aging such asozone, oxidation or UV radiation. On the other hand, it is known thatcertain rubber-making additives, in particular certain coupling agents,are available in a form supported by carbon black, the use of suchadditives therefore involving the incorporation, in a small proportion,of carbon black. Suitable carbon blacks are any carbon blacks, inparticular the blacks of the type HAF, ISAF and SAF, which areconventionally used in tires, and particularly in treads for thesetires; as non-limitative examples of such blacks, mention may be made ofthe blacks N115, N134, N234, N339, N347 and N375.

In the present specification, the BET specific surface area isdetermined in known manner by adsorption of gas using the method ofBrunauer-Emmett-Teller described in “The Journal of the AmericanChemical Society” Vol. 60, page 309, February 1938, more precisely inaccordance with French Standard NF ISO 9277 of December 1996 [multipointvolumetric method (5 points)—gas: nitrogen—degassing: 1 hour at 160°C.—range of relative pressure p/po: 0.05 to 0.17]. The CTAB specificsurface area is the external surface area determined in accordance withFrench Standard NF T 45-007 of November 1987 (method B).

Finally, as filler equivalent to such a reinforcing inorganic filler,there could be used a reinforcing filler of organic type, in particulara carbon black, coated at least in part with an inorganic layer (forexample, a layer of silica), which for its part requires the use of acoupling agent to provide the connection to the elastomer.

II-3. Coupling Agent

In known manner, in the presence of a reinforcing inorganic filler, itis necessary to use a coupling agent or bonding agent, the function ofwhich is to provide a sufficient chemical and/or physical connectionbetween the inorganic filler (surface of its particles) and the dieneelastomer.

Such a coupling agent, which is consequently at least bifunctional, has,for example, the simplified general formula “Y-T-X”, in which:

-   -   Y represents a functional group (“Y” function) which is capable        of bonding physically and/or chemically with the inorganic        filler, such a bond being able to be established, for example,        between a silicon atom of the coupling agent and the surface        hydroxyl (OH) groups of the inorganic filler (for example,        surface silanols in the case of silica);    -   X represents a functional group (“X” function) which is capable        of bonding physically and/or chemically with the diene        elastomer, for example by means of a sulphur atom; and    -   T represents a divalent group making it possible to link Y and        X.

The coupling agents must particularly not be confused with simple agentsfor coating the inorganic filler which, in known manner, may comprisethe function Y which is active with respect to the inorganic filler butare devoid of the function X which is active with respect to theelastomer.

(Silica/diene elastomer) coupling agents, of variable effectiveness,have been described in a very large number of documents and arewell-known to the person skilled in the art. Any coupling agent likelyto ensure, in the diene rubber compositions usable for the manufacturingof tire treads, the effective bonding between a reinforcing inorganicfiller such as silica and a diene elastomer, in particular organosilanesor polyfunctional polyorganosiloxanes bearing the functions X and Y, maybe used.

In particular polysulphurized silanes, which are referred to as“symmetrical” or “asymmetrical” depending on their specific structure,are used, such as those described for example in the patents or patentapplications FR 2 149 339, FR 2 206 330, U.S. Pat. No. 3,842,111, U.S.Pat. No. 3,873,489, U.S. Pat. No. 3,978,103, U.S. Pat. No. 3,997,581,U.S. Pat. No. 4,002,594, U.S. Pat. No. 4,072,701, U.S. Pat. No.4,129,585, U.S. Pat. No. 5,580,919, U.S. Pat. No. 5,583,245, U.S. Pat.No. 5,650,457, U.S. Pat. No. 5,663,358, U.S. Pat. No. 5,663,395, U.S.Pat. No. 5,663,396, U.S. Pat. No. 5,674,932, U.S. Pat. No. 5,675,014,U.S. Pat. No. 5,684,171, U.S. Pat. No. 5,684,172, U.S. Pat. No.5,696,197, U.S. Pat. No. 5,708,053, U.S. Pat. No. 5,892,085, EP 1 043357 or WO02/083782.

Particularly suitable for implementing the invention, without thedefinition below being limiting, are what are called “symmetrical”polysulphurized silanes which satisfy the following general formula (D:Z-A-S_(n)-A-Z,  (I)

-   -   in which:        -   n is an integer from 2 to 8 (preferably from 2 to 5);        -   A is a divalent hydrocarbon radical (preferably C₁-C₁₈            alkylene groups or C₆-C₁₂ arylene groups, more particularly            C₁-C₁₀ alkylenes, notably C₁-C₄ alkylenes, in particular            propylene);        -   Z corresponds to one of the formulae below:    -   in which:        -   the radicals R¹, which may or may not be substituted, and            may be identical or different, represent a C₁-C₁₈ alkyl            group, a C₅-C₁₈ cycloalkyl group or a C₆-C₁₈ aryl group,            (preferably C₁-C₆ alkyl groups, cyclohexyl or phenyl, in            particular C₁-C₄ alkyl groups, more particularly methyl            and/or ethyl); and        -   the radicals R², which may or may not be substituted, and            may be identical or different, represent a C₁-C₁₈ alkoxyl            group or a C₅-C₁₈ cycloalkoxyl group (preferably a group            selected from among C₁-C₈ alkoxyls and C₅-C₈ cycloalkoxyls,            more preferably still a group selected from among C₁-C₄            alkoxyls, in particular methoxyl and/or ethoxyl).

In the case of a mixture of polysulphurized alkoxysilanes in accordancewith Formula (I) above, in particular conventional, commerciallyavailable, mixtures, the average value of the “n”s is a fractionalnumber, preferably between 2 and 5, more preferably close to 4. However,the invention may also be implemented advantageously for example withdisulphurized alkoxysilanes (n=2).

As examples of polysulphurized silanes, mention will be made moreparticularly of the polysulphides (in particular disulphides,trisulphides or tetrasulphides) ofbis-((C₁-C₄)alkoxyl-(C₁-C₄)alkylsilyl(C₁-C₄)alkyl), such as for examplethe polysulphides of bis(3-trimethoxysilylpropyl) or ofbis(3-triethoxysilylpropyl). Of these compounds, in particularbis(3-triethoxysilylpropyl) tetrasulphide, abbreviated TESPT, of theformula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, or bis(triethoxysilylpropyl) disulphide,abbreviated TESPD, of the formula [(C₂H₅O)₃Si(CH₂)₃S]₂, are used.

TESPD is sold, for example, by Degussa under the name Si75 (in the formof a mixture of disulphide—75% by weight—and of polysulphides), oralternatively by Witco under the name Silquest A1589. TESPT is sold, forexample, by Degussa under the name Si69 (or X50S when it is supported to50% by weight on carbon black), or alternatively by Osi Specialtiesunder the name Silquest A1289 (in both cases, a commercial mixture ofpolysulphides having an average value of n which is close to 4).

Mention will also be made as advantageous coupling agent of thepolysulphides (in particular disulphides, trisulphides ortetrasulphides) of bis-(mono(C₁-C₄)alkoxyl-di(C₁-C₄)alkylsilylpropyl),more particularly bis-monoethoxydimethylsilylpropyl tetrasulphide asdescribed in the aforementioned application WO02/083782.

As examples of coupling agents other than the aforementionedpolysulphurized alkoxysilanes, mention will be made in particular of thebifunctional polyorganosiloxanes such as described for example in theaforementioned applications WO99/02602 or WO01/96442, or alternativelythe hydroxysilane polysulphides such as described in the aforementionedapplications WO02/30939 and WO02/31041.

In the treads according to the invention, the content of coupling agentis preferably between 4 and 12 phr, more preferably between 3 and 8 phr.However, it is generally desirable to use as little as possible thereof.Relative to the weight of reinforcing inorganic filler, the amount ofcoupling agent typically represents between 0.5 and 15% by weightrelative to the quantity of reinforcing inorganic filler. In the casefor example of tire treads for passenger vehicles, the coupling agent isused in a preferred amount of less than 12%, or even less than 10% byweight relative to this quantity of reinforcing inorganic filler.

The coupling agent could be grafted beforehand (via the “X” function) onto the diene elastomer of the composition of the invention, theelastomer thus functionalized or “precoupled” then comprising the free“Y” function for the reinforcing inorganic filler. The coupling agentcould also be grafted beforehand (via the “Y” function) on to thereinforcing inorganic filler, the filler thus “precoupled” then beingable to be bonded to the diene elastomer by means of the free “X”function. However, it is preferred, in particular for reasons of betterprocessing of the compositions in the uncured state, to use the couplingagent either grafted onto the reinforcing inorganic filler, or in thefree (i.e. non-grafted) state.

There may possibly be associated with the coupling agent an appropriate“coupling activator”, that is to say, a body (single compound orassociation of compounds) which, when mixed with this coupling agent,increases the effectiveness of the latter. Coupling activators forpolysulphurized alkoxysilanes have for example been described in theaforementioned international applications WO00/05300 and WO00/05301,consisting of the association of a substituted guanidine, in particularN,N′-diphenylguanidine (abbreviated to “DPG”), with an enamine or a zincdithiophosphate. The presence of these coupling activators will make itpossible, for example, to keep the amount of coupling agent at apreferred level of less than 10%, or even less than 8% by weightrelative to the quantity of reinforcing inorganic filler, oralternatively to reduce the amount of reinforcing inorganic filler owingto the improved coupling with the diene elastomer.

II-4. Bismaleimide Compound

The rubber compositions of the treads of the invention contain at least,in an amount of between 4 and 12 phr, a bismaleimide compound intendedto form in situ, after curing (vulcanization) of the tread, athree-dimensional resin lattice which is superposed and interpenetrateswith the (inorganic filler/elastomer) lattice on one hand, and with the(elastomer/sulphur) lattice on the other hand (if the cross-linkingagent is sulphur).

The invention is preferably implemented with bismaleimide compounds ofthe general formula:

in which R is an aromatic or aliphatic, cyclic or acyclic hydrocarbonradical, whether substituted or non-substituted, such a radical possiblycomprising a heteroatom selected from among O, N and S; this radical Rpreferably comprises from 2 to 24 carbon atoms.

Such a definition includes for example N,N′-(ethylene)-bismaleimides,N,N′-(hexamethylene)-bismaleimides,N,N′-(dodecamethylene)-bismaleimides,N,N′-(2,2,4-trimethyl-hexamethylene)-bismaleimides,N,N′-(oxy-dipropylene)-bismaleimides,N,N′-(amino-di-propylene)-bismaleimides,N,N′-(1,3-cyclohexylene)-bismaleimides,N,N′-(1,4-cyclo-hexylene)-bismaleimides,N,N′-(methylene-1,4-dicyclohexylene)-bismaleimides,N,N′-(3,3′-dimethyl-4,4′-biphenylene)-bismaleimides,N,N′-(m-phenylene)-bismaleimides, N,N′-(p-phenylene)-bismaleimides,N,N′-(o-phenylene)-bismaleimides, N,N′-(1,3-naphthylene)-bismaleimides,N,N′-(1,4-naphthylene)-bismaleim ides,N,N′-(1,5-naphthylene)-bismaleimides,N,N′-(4,6-dimethyl-1,3-phenylene)-bismaleimides,N,N′-(2,4-toluylene)-bismaleimides, N,N′-(2,6-toluylene)-bismaleimides,N,N′-(methylene-di-p-phenylene)-bismaleimides,N,N′-(oxy-dipropylene)-bismaleimides,N,N′-(oxy-di-p-phenylene)-bismaleimides,N,N′-(4,4′-di-phenylmethane)-bismaleimides,N,N′-(4,4′-diphenylether)-bismaleimides,N,N′-(4,4′-diphenylsulphone)-bismaleimides andN,N′-(4,4′-diphenyldithio)-bismaleimides.

“Bismaleimide compound” is also understood to mean poly-bismaleimidecompounds (bismaleimide polymers or oligomers).

The bismaleimides preferably selected areN,N′-(m-phenylene)-bismaleimide (abbreviated to “MPBM”) andN,N′-(4,4′-diphenylmethane)-bismaleimide (abbreviated to “DPBM”).

The quantity of bismaleimide compound may vary as a function of thenature of the diene elastomer used and the quantity of reinforcinginorganic filler, within a range of between 4 and 12 phr. Below theminimum indicated, the technical effect desired is not obtained, whereasbeyond the maximum indicated there is excessive stiffening, excessivecompromising of the hysteresis and a significant increase in cost. Aquantity within a range from 5 to 10 phr has proved well suited, inparticular in the case of treads for passenger-vehicle tires.

A radical initiator (free-radical generator), for example an organicperoxide, could possibly be added, in a very small quantity, to therubber compositions to activate the formation of the bismaleimide resinlattice, during their curing.

II-5. Various Additives

Of course, the rubber compositions of the treads according to theinvention also comprise all or part of the additives usually used insulphur-cross-linkable diene rubber compositions intended for themanufacturing of treads, such as, for example, plasticizers, pigments,protective agents of the type antioxidants, antiozonants, anti-fatigueagents, other reinforcing resins, a cross-linking system based either onsulphur or on sulphur and/or peroxide donors, vulcanizationaccelerators, vulcanization activators, extender oils, etc. There mayalso be associated with the reinforcing inorganic filler, if necessary,a conventional non-reinforcing white filler, such as for exampleparticles of clay, bentonite, talc, chalk, kaolin or titanium oxides.

The rubber compositions of the treads of the invention may also contain,in addition to the coupling agents, agents for coating the reinforcinginorganic filler (comprising for example the single function Y), or moregenerally processing aids liable, in known manner, owing to animprovement in the dispersion of the inorganic filler in the rubbermatrix and to a reduction in the viscosity of the compositions, toimprove their processability in the uncured state, these agents, used ina preferred amount of between 0.5 and 3 phr, being, for example,alkylalkoxysilanes, (in particular alkyltriethoxysilanes), polyols,polyethers (for example polyethylene glycols), primary, secondary ortertiary amines, hydroxylated or hydrolyzable polyorganosiloxanes, forexample α,ω-dihydroxypolyorganosiloxanes (in particularα,ω-dihydroxy-polydimethylsiloxanes).

II-6. Manufacturing of the Treads

The rubber compositions of the treads of the invention are manufacturedin suitable mixers, using two successive preparation phases inaccordance with a general process well-known to the person skilled inthe art: a first phase of thermomechanical working or kneading(sometimes referred to as “non-productive” phase) at high temperature,up to a maximum temperature of between 130° C. and 200° C., preferablybetween 145° C. and 185° C., followed by a second phase of mechanicalworking (sometimes referred to as “productive” phase) at lowertemperature, typically less than 120° C., for example between 60° C. and100° C., during which finishing phase the cross-linking or vulcanizationsystem is incorporated.

The process according to the invention, for preparing a tire treadhaving, after curing and mechanical running-in of the tire for which itis intended, a rigidity gradient radially increasing from the surfacetowards the inside of the tread, comprises the following steps:

-   -   incorporating in a diene elastomer, in a mixer, during a first        step referred to as “non-productive”:        -   more than 60 phr of a reinforcing inorganic filler;        -   between 2 and 15 phr of a coupling agent; and        -   between 4 and 12 phr of a bismaleimide compound;    -    thermomechanically kneading the entire mixture, in one or more        stages, until a maximum temperature of between 130° C. and        200° C. is reached;    -   cooling the entire mixture to a temperature of less than 100°        C.;    -   then incorporating, during a second step referred to as        “productive”:        -   a vulcanization system;    -   kneading the entire mixture until a maximum temperature less        than 120° C. is reached; and    -   extruding or calendering the rubber composition thus obtained,        in the form of a tire tread.

According to a preferred embodiment, all the base constituents of thecompositions of the treads according to the invention, with theexception of the vulcanization system and any radical initiator, namelythe reinforcing inorganic filler, the coupling agent and thebismaleimide compound, are incorporated intimately by kneading in thediene elastomer during the first, so-called non-productive, phase, thatis to say that at least these different base constituents are introducedinto the mixer and are kneaded thermomechanically, in one or morestages, until the maximum temperature of between 130° C. and 200° C.,preferably between 145° C. and 185° C., is reached. However, all or partof the bismaleimide compound could also be incorporated during theproductive phase.

By way of example, the first (non-productive) phase is effected in asingle thermomechanical step during which all the necessaryconstituents, any complementary coating agents or processing agents andvarious other additives, with the exception of the vulcanization system,are introduced into a suitable mixer, such as a conventional internalmixer. A second stage of thermomechanical working could possibly beadded, in this internal mixer, for example after an intermediate coolingstage (preferably to a temperature of less than 100° C.), with the aimof making the compositions undergo complementary heat treatment, inparticular in order to improve the dispersion, in the elastomericmatrix, of the reinforcing inorganic filler, the coupling agent and thebismaleimide compound.

After cooling the mixture thus obtained at the end of the first,non-productive, phase, the vulcanization system is then incorporated atlow temperature, in an external mixer such as an open mill. The entiremixture is then mixed (productive phase) for several minutes, forexample between 5 and 15 minutes.

The vulcanization system proper is preferably based on sulphur and aprimary vulcanization accelerator, in particular an accelerator of thesulphenamide type. To this vulcanization system there are added,incorporated during the first, non-productive, phase and/or during theproductive phase, various known secondary accelerators or vulcanizationactivators such as zinc oxide, stearic acid, guanidine derivatives (inparticular diphenylguanidine). The amount of sulphur is preferablybetween 0.5 and 3.0 phr, and the amount of the primary accelerator ispreferably between 0.5 and 5.0 phr.

The final composition thus obtained is then calendered, for example inthe form of a film or a sheet, in particular for characterization in thelaboratory, or alternatively extruded in the form of a rubber profiledelement usable directly as a tire tread.

The vulcanization (or curing) is carried out in known manner at atemperature generally between 130° C. and 200° C., for a sufficient timewhich may vary, for example, between 5 and 90 minutes, depending, inparticular, on the curing temperature, the vulcanization system adoptedand the vulcanization kinetics of the composition in question, and thesize of the tire in question.

To summarize, in the process according to the invention, in accordancewith all the particulars given previously, preferably at least one, morepreferably all, of the following characteriztics are satisfied:

-   -   the amount of reinforcing inorganic filler is greater than 70        phr;    -   the amount of coupling agent is between 4 and 12 phr;    -   the amount of bismaleimide compound is between 5 and 10 phr;    -   the maximum thermomechanical kneading temperature is between        145° C. and 180° C.;    -   the reinforcing inorganic filler is a siliceous or aluminous        filler;    -   the amount of carbon black is less than 20 phr, preferably        between 2 and 15 phr;    -   the at least bifunctional coupling agent is an organosilane or a        polyorganosiloxane;    -   the bismaleimide compound is N,N′-(m-phenylene)-bismaleimide or        N,N′-(4,4′-di-phenyl-methane)-bismaleimide;    -   the diene elastomer is a butadiene/styrene copolymer (SBR),        preferably used in a mixture with a polybutadiene;    -   the reinforcing inorganic filler represents more than 80% by        weight of the total reinforcing filler.

More preferably, in this process, at least one, even more preferablyall, of the following characteriztics are satisfied:

-   -   the amount of inorganic filler lies within a range from 80 to        110 phr;    -   the amount of coupling agent is between 3 and 8 phr;    -   the reinforcing inorganic filler is silica;    -   the amount of carbon black is less than 15 phr, preferably        between 4 and 12 phr;    -   the coupling agent is an organosilane;    -   the bismaleimide compound is        N,N′-(4,4′-diphenylmethane)bismaleimide;    -   the diene elastomer is an SBR prepared in solution used in a        mixture with a polybutadiene having more than 90% (mol) cis-1,4        bonds;    -   the reinforcing inorganic filler represents more than 90% by        weight of the total reinforcing filler.

Of course, the invention relates to the treads previously described,both in the uncured state (i.e. before curing) and in the cured state(i.e. after cross-linking or vulcanization).

II-7. Properties of the Treads

It can easily be confirmed that the use of a large amount (more than 60or more than 70 phr) of reinforcing filler, whatever the type of fillerused (carbon black or inorganic filler), in combination with more than 4phr of bismaleimide compound such as DPBM is accompanied, after curingof the compositions, by a great increase in the moduli at lowdeformation (the values of ME10 possibly being, for example, doubled)and an increase in the Shore A hardness (increased for example by 10 to20%).

Such an increase in rigidity, which was expected, admittedly allowed theperson skilled in the art to predict, for tires mounted on automobilesthe treads of which were constituted by such compositions, animprovement in road behavior owing to increased stiffening and thereforeincreased drift thrust, but also and above all an extremelydisadvantageous drop in the grip performance on wet, snow-covered or icyground.

Such a drop in the grip performance was in fact observed for comparabletreads filled with carbon black, but not in the case of treads accordingto the invention; the latter, surprisingly, benefit from the improvedroad behavior without adversely affecting the grip performance above.

It must of necessity be deduced from this that the resin latticeprovided by the bismaleimide compound, in the rubber compositions of thetreads, is “expressed” differently depending on whether thesecompositions are filled conventionally with carbon black, or on thecontrary with a reinforcing inorganic filler such as silica, in the highamounts advocated.

Complementary tests have revealed an unexpected property for the treadsaccording to the invention, which a posteriori explains the abovedifferences in behavior.

These treads, owing to an unexpected phenomenon of auto-accommodationafter rolling, have a greatly marked rigidity gradient in the radialdirection, this rigidity increasing radially from the surface towardsthe inside of the tread in a continuous profile, over the entirethickness of the tread or at the very least of the portion of hybridtread, if applicable, comprising the composition reinforced withinorganic filler and bismaleimide according to the invention. Such acharacteristic does not exist in the case of a control tread reinforcedwith carbon black and the bismaleimide compound.

From all the above results, it may be thought that the stiffeningthree-dimensional lattice formed by the bismaleimide compound has lessersolidity in the case of the tread filled with silica than in the case ofthe conventional tread filled with carbon black. Due to this relativefragility, stresses of low amplitude, typical of those experiencedduring rolling by the surface part of the tread, would be sufficient tobreak the surface bismaleimide resin lattice, and thus to make thesurface part of the tread more flexible and less rigid, and thus make itrecover the excellent grip performance which it has in the absence ofbismaleimide. On the other hand, in depth, the lattice resin would belittle affected by rolling, all the less so as one penetrates insidethis tread, thus guaranteeing the additional rigidity sufficient forimproved road behavior (greater drift thrust).

Thus, the tread according to the invention, which is flexible at thesurface and rigid in its depth once accommodated (run in), combines thetwo contradictory demands of road behavior and grip on wet, snow-coveredor icy ground.

The rigidity gradient described above is illustrated in particular by avery significant difference in modulus at low deformation, or in Shorehardness, between the radially outermost part of the tread, the part incontact with the ground, and the radially innermost part of this tread(or portion of tread, in the case for example of a hybrid tread)stiffened by the presence of the bismaleimide resin lattice.

After standard running-in of passenger-car tires according to theinvention, the secant modulus ME10 (10% elongation) may thus varyadvantageously from one to two times as much, between the surface(typically between 2 and 8 MPa, depending on the type of tire) and theradially innermost part (typically between 4 and 16 MPa, depending onthe type of tire) of the tread.

Comparative Shore hardness measurements carried out on the surface oftreads whether or not in accordance with the invention typically yieldthe following results: Carbon black filler: without bismaleimide, newtire: between 60 and 70 points; with bismaleimide (6 phr), new tire:between 70 and 80 points; with bismaleimide (6 phr), after standardbetween 70 and 80 points. running-in: Silica filler: withoutbismaleimide, new tire: between 60 and 70 points; with bismaleimide (6phr), new tire: between 70 and 80 points; with bismaleimide (6 phr),after standard between 60 and 70 points. running-in:

The conventional tire, the tread of which is reinforced with carbonblack and bismaleimide retains its surface rigidity after running-in,whereas the tire according to the invention, the tread of which isreinforced with silica and bismaleimide recovers its range of initialhardness after running-in, which confirms the changes in moduli ME10commented on previously.

The invention thus unexpectedly makes it possible to modulate thedifferences in rigidity between the surface of a tread and its radiallyinner part, and therefore to adjust the desired compromise of grip/roadbehavior.

It is henceforth possible to produce tires of low rolling resistance thetread of which (at the very least for the portion of tread comprisingthe bismaleimide compound in the case for example of a hybrid tread ofthe “cap-base” type) has a modulus which increases radially in acontinuous profile, which is low at the surface (for example between 2and 8 MPa in the case of a passenger-car tire), and high in the depth(for example between 8 and 16 MPa in its radially innermost part, forthis same passenger-car tire).

Preferably, in the case of a conventional passenger-car tire (summer),the modulus ME10 will thus be between 5 and 8 MPa (in particular between5.5 and 7.5 MPa) at the surface and between 8 and 14 MPa (in particularbetween 9 and 13 MPa) in the depth (radially innermost part).Preferably, in the case of a “winter” tire intended for snow-covered oricy roads, the modulus ME10 will be between 3 and 6 MPa at the surface(in particular between 3.5 and 5.5 MPa), and between 6 and 12 MPa (inparticular between 7 and 11 MPa) in the depth.

III. Example of Embodiment

III-1. Preparation of the Rubber Compositions and Treads

For the following tests, the procedure is as follows: the reinforcingfiller, the coupling agent, the diene elastomer or the mixture of dieneelastomers, the bismaleimide compound and the various other ingredients,with the exception of the vulcanization system, are introduced insuccession into an internal mixer filled to 70% of capacity, the initialtank temperature of which is approximately 60° C. Thermomechanicalworking (non-productive phase) is then performed in one stage, of aduration of about 3 to 4 minutes in total, until a maximum “dropping”temperature of 165° C. is obtained. The mixture thus obtained isrecovered, it is cooled and then sulphur and sulphenamide acceleratorare incorporated on an external mixer (homo-finisher) at 30° C., bymixing everything (productive phase) for an appropriate time, of between5 and 12 minutes.

The compositions thus obtained are then calendered either in the form ofplates (thickness of 2 to 3 mm) or of thin sheets of rubber in order tomeasure their physical or mechanical properties, or extruded in the formof treads for radial-carcass passenger-car tires (dimensions 195/65R15—speed index H).

III-2. Tests

In this test, 7 rubber compositions are compared, based on known SBR andBR diene elastomers, reinforced by carbon black or silica, usable astreads for summer tires for passenger vehicles.

These compositions C-1 to C-7 are distinguished essentially by thefollowing characteristics:

-   -   C-1: reinforced by 70 phr of carbon black, without bismaleimide;    -   C-2: reinforced by 70 phr of carbon black+6 phr of bismaleimide;    -   C-3: reinforced by 80 phr of silica, without bismaleimide;    -   C-4: reinforced by 80 phr of silica+6 phr of bismaleimide;    -   C-5: reinforced by 80 phr of silica+8 phr of bismaleimide;    -   C-6: reinforced by 80 phr of silica+2 phr of bismaleimide;    -   C-7: reinforced by 60 phr of silica+2 phr of bismaleimide.

The compositions C-1 and C-3 constitute the references “black” and“silica” of this test. Their respective formulations were adjusted so asto bring them both to initial iso-rigidity (Shore A hardness), beforeincorporation of bismaleimide.

The compositions C-3 to C-7 furthermore contain the coupling agent TESPT(amount of 8% by weight relative to the quantity of silica) and the DPG(approximately 2.6% by weight relative to the quantity of silica). Inthese compositions C-3 to C-7, the carbon black is present in a verysmall amount (6 phr), essentially used as a black pigmentation agent.

Only the treads comprising the compositions C-4 and C-5 are therefore inaccordance with the invention. The compositions C-6 and C-7 are not inaccordance with the invention because they comprise an insufficientamount of bismaleimide, whatever the amount of reinforcing inorganicfiller (60 or 80 phr).

Tables 1 and 2 show the formulation of the different compositions (Table1—amounts of the different products expressed in phr), and theirproperties after curing (40 min at 150° C.).

A comparison, first of all, of the control compositions C-1 and C-3(devoid of bismaleimide) demonstrates that after curing they have anequivalent rigidity (Shore A hardnesses identical; values of modulus atlow deformation ME10 very close) and identical reinforcement properties(identical moduli at high deformations—ME100 and ME300).

After incorporation of bismaleimide in a sufficient quantity (6 or 8phr), for both types of compositions (compare C-2 to C-1 on one hand,C-4 and C-5 to C-3 on the other hand) a great rise is observed in thevalue of modulus at low deformation (values ME10 practically doubled)and in the Shore hardness (increased by 10 to 20%).

This stiffening may be described as expected, as explained previously.

However, one significant difference must be noted between compositionsC-2 on one hand and C-4 or C-5 on the other hand, this differencerelating to the evolution of the modulus (ME10_(Ac.)) at lowdeformation, after mechanical accommodation (15%).

In the case of the control composition C-2 (carbon black filler), itwill be noted that the modulus ME10_(Ac.) remains very high afteraccommodation (9.1 MPa compared with an initial 5.5 MPa for compositionC-1, or approximately 65% greater); on the contrary, this same modulusME10_(Ac.) drops very greatly (from 11-12 MPa to 7-7.5 MPa) forcompositions C-4 and C-5 (silica filler), recovering practically theinitial value ME10 (6.0 MPa) recorded for the control composition C-3devoid of bismaleimide. The ratios (ME10_(Ac.)/ME10) of Table 2,expressed in %, clearly express this difference in behavior between thecompositions C-2 and C-4 or C-5.

These results show that the bismaleimide lattice is therefore“expressed” differently, and has a different solidity depending onwhether the compositions are filled with carbon black, or with areinforcing inorganic filler such as silica, in the high amountsadvocated.

As for the other compositions not in accordance with the invention C-6and C-7, comprising a lesser (insufficient) amount of bismaleimide (2phr), it will be noted that the gain in rigidity is too low ornon-existent (see modulus ME10 and Shore hardness).

The rubber compositions previously described, based on diene elastomer,reinforcing inorganic filler, a coupling agent and a bismaleimidecompound, in the proportions indicated above, may advantageouslyconstitute the entire tread according to the invention.

However, the invention also applies to those cases in which these rubbercompositions comprising the bismaleimide compound form only part of acomposite tread such as described for example in the introduction to thepresent specification, formed of at least two radially superposed layersof different rigidity (so-called “cap-base” structure), both intended tocome into contact with the road during rolling of the tire, during thelife of the latter. The part comprising the bismaleimide compound maythen constitute the radially outer layer of the tread intended to comeinto contact with the ground from the start of rolling of the new tire,or on the contrary its radially inner layer intended to come intocontact with the ground later on, in the event that it is for exampledesired to “delay” the technical effect of auto-accommodation providedby the invention.

Due to the treads according to the invention and to the specificformulation of their rubber compositions, it is henceforth possible to“reconcile” grip on wet ground and road behaviour, without usingsolutions which are complex, costly or non-durable such as described inthe introduction to the present specification.

The treads according to the invention offer the major advantage,compared in particular with the composite treads of the prior art, onone hand of maintaining their compromise of performances throughout thelife of the tire, due to the unexpected phenomenon of auto-accommodationwhich is observed, and on the other hand of having a true radialrigidity gradient, and not a simple, very localized, “accident” ofrigidity. This true rigidity gradient results in optimum “working” ofthe blocks of rubber in contact with the ground, during rolling and thenumerous forces transmitted to the tread, in other words is synonymouswith a tire which grips the road even better.

This result may be obtained while maintaining the performances ofrolling resistance and wear resistance at the high levels which one isentitled to expect nowadays from rubber compositions based onreinforcing inorganic fillers such as highly dispersible silicas.

The invention finds a very advantageous application in tires fitted onvehicles such as motorcycles, passenger cars, vans or heavy vehicles, inparticular in high-grip tires of the “snow” or “ice” type (also referredto as “winter” tires) which, owing to a deliberately more flexibletread, could have lower-performance road behaviour on dry ground. TABLE1 Composition No.: C-1 C-2 C-3 C-4 C-5 C-6 C-7 SBR (1) 70 70 70 70 70 7070 BR (2) 30 30 30 30 30 30 30 carbon black (3) 70 70 6 6 6 6 6 silica(4) — — 80 80 80 80 60 coupling agent (5) — — 6.4 6.4 6.4 6.4 6.4 MPBM(6) — 6 — 6 8 2 2 oil (7) 20 20 39 39 39 39 39 DPG (8) — — 2.1 2.1 2.12.1 2.1 ZnO 2.5 2.5 2.5 2.5 2.5 2.5 2.5 stearic acid 2 2 2 2 2 2 2antioxidant (9) 1.9 1.9 1.9 1.9 1.9 1.9 1.9 sulphur 1.5 1.5 1.5 1.5 1.51.5 1.5 accelerator (11) 1.5 1.5 1.5 1.5 1.5 1.5 1.5(1) SBR (expressed in dry SBR) extended with 37.5% by weight (26.25 phr)of oil (or a total of 96.25 phr of extended SBR); 26.5% styrene, 59.5%1,2-polybutadiene units and 23% trans-1,4-polybutadiene units (Tg = −29°C.);(2) BR with 4.3% of 1, 2; 2.7% of trans; 93% of cis 1, 4 (Tg = −106°C.);(3) carbon black N234;(4) silica “Zeosil 1165 MP” from Rhodia, type “HDS” (BET and CTAB:approximately 160 m²/g);(5) TESPT coupling agent (“Si69” from Degussa);(6) N,N′-(m-phenylene)-bismaleimide (“HVA2”- from Du Pont de Nemours);(7) total aromatic oil (including extender oil for the SBR);(8) diphenylguanidine (Perkacit DPG from Flexsys);(9) N-1,3-dimethylbutyl-N-phenylparaphenylenediamine (Santoflex 6-PPDfrom Flexsys);(10) HMT (from Degussa);(11) N-cyclohexyl-2-benzothiazyl sulphenamide (Santocure CBS fromFlexsys).

TABLE 2 Composition No. C-1 C-2 C-3 C-4 C-5 C-6 C-7 Shore A 65 73 66 7377 69 66 Shore A (r. u.) 100 112 100 111 117 105 100 ME10 (MPa) 5.5 11.86.0 12.0 11.5 7.0 5.9 ME10 (r. u.) 100 210 100 200 190 120 100 ME100(MPa) 1.85 2.3 1.9 2.2 2.4 2.1 1.8 ME300 (MPa) 2.2 2.0 2.15 1.6 2.1 2.11.8 ME10_(Ac.) 4.4 9.1 4.8 7.2 7.4 — — ME10_(Ac.) ^(/ME10) 80% 79% 80%60% 64% — —(r. u. relative units)

1. A tire tread comprising a rubber composition based on at least: (i) adiene elastomer; (ii) more than 60 phr of a reinforcing inorganicfiller; (iii) between 2 and 15 phr of a coupling agent; and (iv) between4 and 12 phr of a bismaleimide compound.
 2. The tread according to claim1, wherein the diene elastomer is selected from the group which consistsof polybutadienes, synthetic polyisoprenes, natural rubber, butadienecopolymers, isoprene copolymers and mixtures of these elastomers.
 3. Thetread according to claim 1, wherein the reinforcing inorganic filler isa siliceous or aluminous filler.
 4. The tread according to claim 3,wherein the reinforcing inorganic filler is silica.
 5. The treadaccording to claim 1, wherein the quantity of reinforcing inorganicfiller is greater than 70 phr.
 6. The tread according to claim 5,wherein the quantity of reinforcing inorganic filler is between 70 and120 phr.
 7. The tread according to claim 1, wherein the bismaleimidecompound is N,N′-(m-phenylene)-bismaleimide orN,N′-(4,4′-diphenylmethane)-bismaleimide.
 8. The tread according toclaim 7, wherein the bismaleimide compound isN,N′-(4,4′-diphenylmethane)-bismaleimide.
 9. The tread according toclaim 1, wherein the amount of bismaleimide compound is within a rangefrom 5 to 10 phr.
 10. The tread according claim 1, wherein the dieneelastomer is a butadiene/styrene copolymer (SBR).
 11. The treadaccording to claim 10, wherein the SBR elastomer has a styrene contentbetween 20% and 30% by weight, a content of vinyl bonds of the butadienefraction between 15% and 65%, a content of trans-1,4 bonds between 20%and 75% and a glass transition temperature between −20° C. and −55° C.12. The tread according to claim 10, wherein the rubber compositionfurther comprises a polybutadiene having more than 90% cis-1,4 bonds.13. The tread according to claim 1, wherein the rubber compositionfurther comprises a carbon black present in an amount less than 20 phr.14. The tread according to claim 13, wherein the amount of carbon blackis between 2 and 15 phr.
 15. The tread according to claim 1, whereinsaid tread comprises at least two different, radially superposed, rubbercompositions, the rubber composition comprising the bismaleimidecompound forming the radially outer part of this tread.
 16. The treadaccording to claim 1, wherein said tread comprises at least twodifferent, radially superposed, rubber compositions, the rubbercomposition comprising the bismaleimide compound forming the radiallyinner part of this tread.
 17. The tread according to claim 1, whereinsaid tread is in the vulcanized state.
 18. A process for preparing atire tread having, after curing and mechanical running-in of the tirecomprising it, a rigidity gradient which increases radially from thesurface towards the inside of the tread, wherein said process comprisesthe following steps: incorporating in a diene elastomer, in a mixer,during a first step referred to as “non-productive”: more than 60 phr ofa reinforcing inorganic filler; between 2 and 15 phr of a couplingagent; between 4 and 12 phr of a bismaleimide compound, bythermomechanically kneading the entire mixture, in one or more stages,until a maximum temperature of between 130° C. and 200° C. is reached;cooling the entire mixture to a temperature of less than 100° C.; thenincorporating, during a second step referred to as “productive”: avulcanization system; kneading the entire mixture until a maximumtemperature less than 120° C. is reached; and extruding or calenderingthe rubber composition thus obtained, in the form of a tire tread. 19.The process according to claim 18, wherein the diene elastomer isselected from among the group consisting of polybutadienes, syntheticpolyisoprenes, natural rubber, butadiene copolymers, isoprene copolymersand mixtures of these elastomers.
 20. The process according to claim 18,wherein the reinforcing inorganic filler is a siliceous or aluminousfiller.
 21. The process according to claim 20, wherein the reinforcinginorganic filler is silica.
 22. The process according to claim 18,wherein the quantity of reinforcing inorganic filler is greater than 70phr.
 23. The process according to claim 22, wherein the quantity ofreinforcing inorganic filler is of between 70 and 120 phr.
 24. Theprocess according to claim 18, wherein the bismaleimide compound isN,N′-(m-phenylene)-bismaleimide orN,N′-(4,4′-diphenylmethane)-bismaleimide.
 25. The process according toclaim 24, wherein the bismaleimide compound isN,N′-(4,4′-diphenylmethane)-bismaleimide.
 26. The process according toclaim 18, wherein the amount of bismaleimide compound is within a rangefrom 5 to 10 phr.
 27. A tire comprising a tread according to any one ofclaims 1 to
 17. 28. A tire according to claim 27, wherein the tire is apassenger-car tire, wherein the tread of said tire has, aftervulcanization and mechanical running-in of the tire, a rigidity gradientwhich increases radially from the surface towards the inside of thetread, the modulus ME10 (secant modulus at 10% elongation) of said treadbeing between 2 and 8 MPa at the surface of said tread, and between 8and 16 MPa in the radially innermost part of said tread.
 29. A tireaccording to claim 28, wherein the tire is a summer passenger-car tire,the modulus ME10 of the tread being between 5 and 8 MPa at the surfaceof said tread, and between 8 and 14 MPa in the radially innermost partof said tread.
 30. A tire according to claim 28, wherein the tire is awinter passenger-car tire, the modulus ME10 of the tread being between 3and 6 MPa at the surface of said tread, and between 6 and 12 MPa in theradially innermost part of said tread.